In automobile bodywork construction, hot or cold-rolled sheets made of steel are used which for reasons of corrosion protection are surface-treated. The demands made on such sheets are highly varied. On the one hand, they should be capable of being easily formed, while on the other they should be of high strength. The high strength is achieved by the addition to iron of specific alloy constituents, such as Mn, Si, Al, and Cr.
In order to optimize the properties profile of high strength steels, it is usual to anneal the sheets immediately before the coating with zinc and/or aluminum in the melting bath. While the hot-dip coating of steel strips which contain only small proportions of the alloy constituents referred to is not problematic, difficulties do arise with the hot-dip coating of steel sheet with higher proportions of alloys using conventional methods. Thus, areas occur, for example, in which the coating only adheres inadequately to the individual steel sheet, or which remain entirely uncoated.
In the prior art there has been a large number of attempts to avoid these difficulties. It appears, however, that an optimum solution to the problem has not yet been achieved.
With a known method of hot-dip coating of a strip of steel with zinc, the strip which is to be coated runs through a directly-heated pre-heater (DFF=Directly Fired Furnace). By changing the gas-air mixture at the gas burners used, an increase in the oxidation potential can be created in the atmosphere surrounding the strip. The increased oxygen potential leads to an oxidation of the iron on the surface of the strip. The iron oxide layer formed in this way is reduced in a following furnace stretch. A specific adjustment of the oxide layer thickness on the surface of the strip is very difficult. At high strip speed it is thinner than at low strip speed. In consequence, no clearly defined condition of the strip surface can be produced in the reducing atmosphere. This can in turn lead to adherence problems of the coating to the strip surface.
In modern hot-dip coating lines with an RTF pre-heater (RTF=Radiant Tube Furnace), by contrast with the known system described heretofore, no gas-heated burners are used. Accordingly, pre-oxidation of the iron by a change in the gas-air mixture cannot take place. Rather, in these systems the complete annealing treatment of the strip takes place in an inert gas atmosphere. With such an annealing treatment of a strip made of steel with elevated proportions of alloy constituents, however, these alloy constituents can form diffused oxides on the strip surface which in this case cannot be reduced. These oxides prevent a perfect coating with zinc and/or aluminum in the melting bath.
In the patent literature too, various different methods of hot-dip coating of a steel strip with different coating materials are described.
For example, from DE 689 12 243 T2 a method is known for the continuous hot-dip coating of a steel strip with aluminum, in which the strip is heated in a continuous furnace. In a first zone, surface impurities are removed. To do this, the furnace atmosphere has a very high temperature. However, because the strip runs through this zone at very high speed, it is only heated to about half the temperature of the atmosphere. In the succeeding second zone, which is under inert gas, the strip is heated to the temperature of the coating material aluminum.
In addition to this, from DE 695 07 977 T2 a two-stage hot-dip coating method is known of an alloyed steel strip containing chrome. According to this method, the strip is annealed in a first stage in order to obtain iron enrichment on the surface of the strip. The strip is then heated in a non-oxidizing atmosphere to the temperature of the coating metal.
From JP 02285057 A the principle is also known of zinc coating a steel strip in a multi-stage method. To do this, the pre-cleaned strip is treated in a non-oxidizing atmosphere at a temperature of about 820° C. The strip is then treated at some 400° C. to 700° C. in a weakly oxidizing atmosphere, before it is reduced on its surface in a reducing atmosphere. The strip, cooled to some 420° C. to 500° C. is then galvanized in the usual manner.